Overall, I am not too impressed with Ning. It seems like it'd be a really cool networking site...if you had enough people interested in joining your network. Problems I can see people running into on this site is not wanting to join it due to already having the more popular Facebook or MySpace, etc.
I have to admit, though, the one positive is that if you are a member of one of these networks, there is a wide variety of methods for you to share and gather information related to your network.
One interesting network I found was for the band Jet (one of Oliver's favorites). They had most of their music videos posted, tour information, band info, etc. So things like that could be pretty cool.
Over all, though, I think I'll stick to my Facebook (eventhough I don't use it very much).